1. Field of the Invention
The present invention relates to an ink jet printer, and more particularly to a color ink jet printer in which occurrence of dot shift is substantially eliminated.
2. Description of the Related Art
In a color ink jet printer of the type having two print heads, ink droplets ejected from the nozzles in the corresponding locations of the two heads will be shifted in the widthwise direction of a print paper if the print heads are not accurately positioned as shown in FIG. 1(D). In FIG. 1(D), denoted by reference numeral 25a is a nozzle array formed in one head for ejecting black ink, and denoted by reference numeral 25b is a nozzle array formed in the other head for ejecting magenta ink.
To solve the above-described problem, the conventional printer employs an ink jet head formed with sixty-six (66) nozzles for each color ink in which sixty-four (64) nozzles out of sixty-six (66) are used for printing. By the inclusion of two superfluous nozzles, it is contemplated to reduce the amount of dot shift in a manner to be described below.
FIG. 2(A) shows black (K) and magenta (M) dot arrays printed by the two heads each formed with sixty-four nozzles. FIG. 2(B) shows the similar dot array patterns printed by the heads each formed with sixty-six nozzles. As shown in FIG. 2(A), the #1 magenta dot is downwardly shifted L1 from the black dot in the corresponding location, i.e , #1. When the dots are printed with the heads having sixty-four nozzles using all the nozzles, such a large dot shift L1 is liable to occur due to the head assembling reasons.
However, with the heads having sixty-six nozzles, the dot shift L2 between the #0 magenta dot and the #1 black dot is smaller than L1. Therefore, for the dot shift pattern as shown in FIG. 2(B), the conventional method uses the #1 to #64 black nozzles and #0 to #63 magenta nozzles but does not use #0 and #65 black nozzles and #64 and #65 magenta nozzles, For the dot shift pattern opposite to that shown in FIG. 2(B), that is, the magenta dot array is printed upward relative to the black dot array, the #1 to #64 magenta nozzles and #0 to #63 black nozzles are used for printing but #0 and #65 magenta nozzles and #64 and #65 black nozzles are not used therefor.
However, the dot shift reducing contemplation with the use of such heads is not satisfactory for a recent color ink jet printer that requires high precision printing capability.
It is an object of the present invention to provide an ink jet printer that is capabale of printing images, characters, symbols and the with high precision. The ink jet printer of the invention employs two or more separate heads mounted on a carriage with nozzle arrays being formed in each of the heads.
To achieve the above and other objects, there is provided an ink jet printer that includes: a carriage motor, a head assembly, a paper feed motor, driving means, and control means. The head assembly includes a carriage and two or more heads mounted on the carriage. The carriage is operatively connected to the carriage motor and reciprocally moved back and forth in a main scanning direction to perform successive scans. Each of the two or more heads is formed with at least one nozzle array having a plurality of nozzles aligned at an equi-pitch in an auxiliary scanning direction perpendicular to the main scanning direction. To print dots on a plurality of dot lines, a series of scans are performed. The paper feed motor is provided for moving a print paper in the auxiliary scanning direction. The driving means is provided for driving the two or more heads to eject ink droplets from the plurality of nozzles. The driving means is provided for driving different heads individually during different scans. During each scan, each of the two or more heads completes printing of dots on a zone determined by a position in the auxiliary scanning direction of the each of the two or more heads mounted on the carriage. The control means is provided for controlling the paper feed motor to move the print paper a relevant amount each time the each of the two or more heads completes printing of the dots on the zone. A memory may further be provided for storing first set data. The first set data contains data regarding paper feed amount of the print paper to be moved by the paper feed motor each time the each of the two or more heads completes printing of the dots on the zone. The control means controls the paper feed motor based on the first set data. The first set data further contains data regarding which head among the two or more heads is to be actuated first among others in a first scan of the series of scans. The driving means firstly drives the head designated by the first set data in the first scan of the series of scans. An amount of movement of the print paper each time the each of the two or more heads completes printing of the dots on the zone is determined on a head basis.
According to another aspect of the invention, there is provided an ink jet printer that includes; a carriage motor, a head assembly, a paper feed motor, a memory, driving means, and control means. The head assembly has a similar structure, however, each of the heads is formed with at least one nozzle array having a plurality of nozzles and at least one superfluous nozzle. The memory stores first set data regarding paper feed amount to be moved by the paper feed motor each time the each of the two or more heads completes printing of the dots on the zone and second data regarding nozzles selected from the plurality of nozzles and at least one superfluous nozzle in each of the two or more heads. The nozzles are selected so that alignment errors of the nozzles in the two or more heads in the main scanning direction become minimum. The selected nozzles are used for ejecting ink droplets from the each of the two or more heads. The control means controls the paper feed motor based on the first set data. The driving means drives the two or more heads to eject ink droplets from the selected nozzles. The control means controls the paper feed motor to move the print paper each time the each of the two or more heads completes printing of the dots on the zone.